An active device such as transistors requires a direct-current (DC) power for operation. A bias circuit is used to supply the DC power to the active device while isolating transmission paths of an alternating-current (AC) signal from a DC power supply.
For example, in an amplifier, an amplification device outputs the amplified AC signal at an output terminal. The main energy of the amplified signal comes from the DC power supply. That is, the amplifier converts the energy received from the DC power supply to that of the AC signal. The bias circuit serves to pass the energy from the DC power supply to the amplification device. In order not to affect the transmission of the AC signal, the bias circuit has a filtering function for preventing the AC signal from flowing into the bias circuit.
FIG. 1 shows an exemplary configuration of a conventional bias circuit 900 that supplies a DC voltage to an active device (FET) 180. A bias supply terminal 800 of the bias circuit 900 is connected to a line between an AC signal source 1 and the active device 180 (between a port A and a port B). The bias circuit 900 includes a parallel resonant circuit 78p with a capacitor 7 having a capacitance C1 and an inductor 8 having an inductance L1 connected in parallel with each other. One end of the parallel resonant circuit 78p corresponds to the bias supply terminal 800 and the other end of the parallel resonant circuit 78p corresponds to a terminal 600 where a DC circuit part 10 with a DC circuit 5, such as a choke coil, and a DC power supply 6 is connected. The DC power supply 6 generates a certain DC voltage with respect to the ground potential. A capacitor 4 having a sufficiently-large capacitance and grounded at one end thereof is connected to the terminal 600. The energy of the DC power supply 6 is supplied to the active device 180 via the terminal 600 and the bias supply terminal 800.
The resonance frequency of the parallel resonant circuit 78p denoted by a character f is determined by the capacitance C1 and the inductance L1. At the resonance frequency f, the impedance of the bias circuit 900 as viewed from the bias supply terminal 800 toward the DC power supply 6 is considered infinite, so that no AC signal enters the bias circuit. In other words, the bias circuit 900 is a bias circuit that operates at the resonance frequency f. In general, the filtering function of the resonant circuit of the conventional bias circuit 900 or the like is effective only in the proximity of the resonant frequency.
However, recently, there is an increasing demand for radio frequency (RF) circuits capable of multiband operation. The amplifier is an essential component of the RF circuits. That is, the amplifier also is desired to operate in multiple bands. Several methods for enabling multiband operation of the amplifier have been proposed. One example is an amplifier capable of simultaneously amplifying signals in two inherent frequency bands (see “Dual-band Power SiMOSFET Amplifier with Two-section Impedance Transformers”, Uchida et al., 2004 IEICE General Conference, C-2-39, referred to as Non-Patent literature 1 hereinafter). The multi-band amplifier can be applied to a system capable of high-speed transmission using a plurality of frequency bands, such as spectrum aggregation. To supply energy from a DC power supply to such a multiband amplifier, the bias circuit itself has to operate in multiple bands. This means that the conventional bias circuit 900 is difficult to apply to the multiband amplifier. Thus, the multi-band amplifier disclosed in Non-Patent literature 1 has a bias circuit that comprises a parallel resonant circuit and a transmission line Q having an effect similar to that of the parallel resonant circuit connected in series with each other. This bias circuit is designed in such a manner that the resonance frequency of the parallel resonant circuit is a first frequency and the transmission line Q is a quarter-wave line that is short-circuited at the tip at a second frequency. Therefore, this bias circuit can supply energy of a DC power supply to the multiband amplifier simultaneously at two different frequency bands. However, it is difficult for the bias circuit configured in this way to supply energy of the DC power supply to the multiband amplifier simultaneously at three or more different frequency bands.
Another known bias circuit capable of multiband operation is a bias circuit disclosed in Japanese Patent Application Laid-Open No. 2006-254378. This bias circuit supplies energy of a DC power supply separately for a plurality of frequency bands by switching and cannot supply energy from a DC power supply simultaneously for a plurality of frequency bands.